This invention relates to an optical disc device and, in particular, relates to focus control of an optical disc having multiple recording layers.
Recent optical discs have increased their storage capacity as CDs (Compact Discs), DVDs (Digital Versatile Discs), BDs (Blu-ray Discs), and the like come into practical use. The increase in storage capacity of these optical discs has been achieved by increasing recoding density. The recording density is increased by reducing a spot size, which is achieved by shortening the wavelength of laser light. Since BDs, it has been difficult to shorten the wavelength of laser light to increase the storage capacity as materials for recording layers of optical discs and optical systems have limitations. To still increase the capacity, multilayer optical discs have been developed and known in which a plurality of recording layers are stacked or multilayered to increase the storage capacity (for example, refer to JP 2009-015893 A, JP 2008-084504 A, and JP 2008-234220 A).
The above-mentioned JP 2009-015893 A relates to an optical disc device that changes the frequency or amplitude of a high frequency signal which is to be superimposed on laser light after focusing on a destination layer (a target layer) to shift focus from layer to layer. The disclosed technique is to determine the frequency or the amplitude of the high frequency signal for each layer at which crosstalk caused by reflection from other layers (inter-layer crosstalk) will be a minimum.
In the above-mentioned JP 2008-084504 A, the power of laser light is changed for every layer if the numerical aperture (NA) for reading is different depending on the layer, namely, if the spot size of laser light is different depending on the layer. Furthermore, in focusing, the high frequency signal superimposed on the laser light is temporarily cut off to prevent deterioration of existing recorded data on the layers other than the target layer or the recording films thereof by the peak power of the superimposed high frequency signal.
The above-mentioned JP 2008-234220 A discloses a technique that preliminarily records read power for every layer in a predetermined area of an optical disc and retrieves the read power from the area to adjust the power of laser light.